1. Field of the Invention
The present invention relates to a printing head and the manufacture method thereof. In particular, the invention relates to a method of manufacturing a printing head connecting an electrode terminal of a printing element substrate to the corresponding lead, and a printing head manufactured by the method.
2. Description of the Related Art
In recent years, an ink jet printing apparatus for ejecting ink droplets from a printing head to perform a printing operation has been increasingly diffused. Such an ink jet printing apparatus is advantageous in that the miniaturization is easy and a color printing can be performed relatively easy for example.
Among methods of manufacturing a printing head used for an ink jet printing apparatus, a method is known in which an electrode pad as an electrode terminal is electrically connected via a stud bump to a flexible film wiring substrate. According to this method, via the bump provided on the electrode pad, the electrode pad is electrically connected to a wiring formed on the flexible film wiring substrate such as TAB or FPC.
FIG. 4A and FIG. 4B illustrate an example of an ink jet printing head using such a flexible film wiring substrate. FIG. 4A is a plan view of the printing head and FIG. 4B is a cross-sectional view taken along the line B-B of FIG. 4A.
In FIG. 4A and FIG. 4B, the reference numeral 101 denotes a printing element substrate made of silicon or the like. A printing element substrate 101 is cut out from a wafer by dicing. The reference numeral 102 denotes a flexible film wiring substrate having an inner lead 105. The inner lead 105 functions as a lead in an electric wiring pattern in the flexible film wiring substrate 102. The printing element substrate 101 and the flexible film wiring substrate 102 are positioned on a supporting member 110 with a high accuracy. The flexible film wiring substrate 102 includes a rectangular device hole 103 for fixing the printing element substrate 101. On the upper face of the flexible film wiring substrate 102, a flat plate-like base film 104 is formed that consists of insulating resin such as polyimide. The inner lead 105 is obtained by bonding the lower face of the base film 104 to a metal foil consisting of conductive material such as a copper foil to subsequently pattern the metal foil to have a desired shape by a photolithography technique. The surface of the inner lead 105 after the patterning is subjected to plating processing by gold or tin or solder or the like. A region of the metal face of the inner lead 105 in which the surface should not be exposed is covered and protected by resist layer 108 or the like. At the same time, a wiring electrode and an electrode pad connected to a main body or the like are also formed.
The inner lead 105 is formed to extend from the flexible film wiring substrate 102 to the opening of the device hole 103. The surface of the printing element substrate 101 has thereon a plurality of electrode pads 106. The electrode pads 106 are electrically connected, via the stud bumps 107, to the tip ends of the inner leads 105 extending to and existing into the opening of the device hole 103.
The electrode pad 106 has thereon the stud bump 107 formed by metal in advance and is connected to the stud bump 107. The connection among the respective terminals by the stud bumps 107 is carried out by fusing the connecting sections of the stud bump 107 to subsequently solidifying the connecting sections while the stud bumps 107 being connected to the respective terminals to achieve the integration therebetween.
The stud bump is arranged on the electrode pad of a printing element substrate when the printing element substrate is in a wafer status and the stud bump is connected to the electrode pad. When the stud bump is arranged, electricity is discharged to a gold wire having a diameter of several tens of microns to form the tip end of the gold wire to have a ball-like shape. Then, the ball-like-shaped tip end of the gold wire is separated from the gold wire to be placed on the electrode pad by receiving the ultrasonic oscillation via a hump tool while the stud bump is being placed on the electrode pad. As described above, there is a method called a single point bonding method by which gold balls are formed one by one on an electrode pad. There is another method for forming a stud bump on an electrode pad called a gang bonding method by which gold plating is used to simultaneously form bumps on all electrode pads of a printing element substrate. When these methods are used to provide the stud bump 107 on the electrode pad, the inner lead 105 to be connected is positioned at a position corresponding to the stud bump 107. Then, during this status, a flexible film wiring substrate is adhered and fixed to the supporting member 110. Thereafter, a bonding tool is used from above the inner lead 105 to join the inner lead 105 to the stud bump 107. As a result, the inner lead 105 is electrically connected to the electrode pad 106. The connection method as described above is called an Inner Lead Bonding (ILB).
With regard to the ILB method, the two methods of the single point bonding method and the gang bonding method are well-known. In any of these ILB methods, at least any of the inner lead 105 and the stud bump 107 is heated at a high temperature and is connected to each other while being fused. When the connection therebetween is performed by the gang bonding method using the inner lead 105 having a gold-plated surface and the stud bump 107 formed by a gold pole, the bonding tool must be heated to a temperature of about 500 degrees C. Furthermore, when the connection therebetween is performed by the single point bonding method as described above, the bonding tool must be heated to a temperature of about 200 degrees C.
When the stud bump and the inner lead once heated to have a high temperature and are subsequently cooled to have an ordinary temperature, the connecting part that has once expanded by the high temperature is cooled and shrinks. When the members to be connected are compared to each other with regard to thermal expansion coefficient, the base film 104 mainly composed of insulating organic resin and the inner lead 105 mainly composed of copper have thermal expansion coefficients that are significantly higher than the thermal expansion coefficient of the printing element substrate 101 composed of silicon or the like. Thus, when the stud bump 107 and the inner lead 105 formed on the electrode pad 106 of the printing element substrate 101 are connected to each other while being heated and then are cooled until an ordinary temperature is reached, there is a risk where a stress may be caused after the cooling and the stress may act on the respective connecting parts.
When this stress exceeds the joint strength between the electrode pad 106 and the stud bump 107 or the joint strength between the stud bump 107 and the inner lead 105, the joint part may be peeled. Specifically, a risk may be caused where this stress may deteriorate the reliability of the connecting part between the inner lead 105 and the electrode pad 106.
In the case of the mounting in an apparatus requiring a high position accuracy such as an ink jet printing head in particular, the printing element substrate 101 must be fixed to the supporting member 110 accurately. Thus, when the residual stress as described above remains in the connecting section, there is a risk where the position accuracies of the respective members may not be maintained to be high.
In order to solve the disadvantage as described above, Japanese Patent Laid-Open No. 2005-101546 discloses a printing head as shown in FIGS. 5A to 5C in which an inner lead is formed to have an easily-deformable shape. By forming the inner lead to have an easily-deformable shape, even when a stress is generated, the inner lead elastically deforms to absorb the stress. This consequently can suppress this stress from causing the deteriorated reliability of the connecting section. This also can consequently suppress the stress from causing the deteriorated position accuracies of the members.
In the case of the printing head disclosed in Japanese Patent Laid-Open No. 2005-101546, however, in order to provide the deformable-shape, the width of the inner lead is partially reduced to reduce the cross-sectional area of a region other than the part at which the inner lead is joined to the electrode pad. Such a reduction of the cross-sectional area of a region other than the part at which the inner lead is joined to the stud bump by the reduced width of the inner lead causes the part to have a higher electric resistance. Due to this reason, when the printing operation is performed, this part is heated whenever power is distributed to the printing element in the ink jet printing head in order to drive the printing element. This may cause, when the printing operation is performed for a long time, excessive heat in the inner lead. Also, due to the repeated deformation of the heated inner lead, there is a possibility that the durability of the inner lead is deteriorated. As described above, even when the stress in the electric connecting part is absorbed by the deformation of the inner lead, the repeated deformation of the inner lead itself may cause a risk of a deteriorated durability of the inner lead. Furthermore, the deformable and complicated shape of the inner lead also may cause a risk where the inner lead may be unintentionally deformed in a step of mounting the flexible film wiring substrate. Furthermore, the complicated shape of the inner lead also may cause a risk of an increased cost for manufacturing the flexible film wiring substrate.
Furthermore, more ink jet printing apparatuses have been miniaturized in recent years. In accordance with this, printing element substrates have been miniaturized and highly-integrated. This proportionally reduces the size of the electrode pad and the pitch between electrode pads, thus proportionally reducing the width of an inner lead. Therefore, there is a risk of further-increased heat generated in the inner lead during power distribution.
On the other hand, it is known that the decreased reliability of the electrode pad is related to not only the difference in thermal expansion between the inner lead and the flexible film wiring substrate but also the direction of the ultrasonic oscillation applied to the connecting section.
FIG. 6A is a plan view illustrating a wafer in which a plurality of printing element substrates are arranged, before a stud bump is placed and connected to a printing element substrate. FIG. 6B is a plan view illustrating electrode pads formed on the printing element substrate and stud bumps formed on the electrode pads. FIG. 6C is a cross-sectional view taken along the line VIC-VIC of the stud bumps placed on the printing element substrate. FIG. 7A is a plan view illustrating the printing element substrate in which the direction of ultrasonic oscillation is shown. FIG. 7B is a cross-sectional view illustrating the stud bumps 107 taken along the line VIIB-VIIB in which the upper parts of the stud bumps 107 are smoothed by the ultrasonic oscillation. FIG. 8 is a plan view illustrating a joint part at which the stud bump is joined to the inner lead by ILB.
As shown in FIG. 6B, by the single point method, the stud bumps 107 are formed on and are joined to the electrode pads 106 of the printing element substrate 101. During this process, the ultrasonic oscillation is applied to the stud bumps 107 in the longitudinal direction of the printing element substrate 101. Thereafter, leveling is performed in order to smooth the uppermost parts of the stud bumps 107. During this process, as in the step of arranging the stud bumps on the inner leads, ultrasonic oscillation is also applied to the stud bumps 107 in the longitudinal direction of the printing element substrate 101 as shown in FIG. 7A. Thereafter, the inner leads 105 and the stud bumps 107 are joined by ILB. During the process of joining the stud bumps 107 and the inner leads 105 by ILB, ultrasonic oscillation is applied to the stud bumps 107 in the longitudinal direction of the printing element substrate 101 as shown in FIG. 8.
Through these steps, the printing element substrate 101 is electrically connected to the flexible film wiring substrate 102 by ultrasonic oscillation. This ultrasonic oscillation performed for the electrical connection is generally performed only in the longitudinal direction of the printing element substrate 101 in all steps. Thus, there is a possibility where the electrode pads of the printing element substrate 101 may receive a relatively-high stress due to the stress caused by the cooling after the joint by the ultrasonic oscillation combined with the above-described stress in the connecting section due to the difference in thermal expansion. Due to the reasons, even when the inner lead has an easily-deformable shape for absorbing the stress left in the cooling process, there is a risk where the total stress may exceed the stress that can be absorbed by the deformation of the shape of the inner lead, thereby deteriorating the reliability of the printing head.